This invention relates to a classifier for centrifugally separating powder such as toner powder used in a copier, or inorganic compounds such as metal oxides, glass and ceramics into fine and coarse particles.
FIG. 6 shows a conventional classifier. It includes a plurality of pivotable louvers 33 provided around a classifying chamber 32 defined between a classifying cover 30 and a classifying plate 31. Defined between the adjacent louvers 33 are passages through which secondary air is introduced into the classifying chamber 32 in a whirling flow. A powder supply tube 34 is provided over the classifying cover 30 to define a powder supply port 35 between the bottom inner edge of the tube 34 and the outer edge of the classifying cover 30.
A fine powder discharge tube 36 is connected to the center of the classifying plate 31, while a coarse powder discharge port 37 is provided around the classifying plate 31.
In operation, while a suction force from a blower is applied to the fine powder discharge tube 36, a fluid mixture of powder and compressed air is supplied into the top of the powder supply tube 34 so that the mixture will be supplied into the classifying chamber 32 through the powder supply port 35 in a whirling flow. At the same time, secondary air is blown into the classifying chamber 32 through the passages between the louvers 33 to increase the whirling speed of the mixture, thereby centrifugally classifying the powder so that finer particles will move toward the center of the chamber 32 to be discharge through the fine powder discharge tube 36, while coarser particles will whirl along the outer periphery of the chamber 32 to be discharge through the coarse powder discharge port 37.
In the powder classification in which a classifier of the above-described type is used, especially in the manufacture of fine powder such as ceramic powder used as material for electronic parts, the lower the classification point and the nearer the maximum particle diameter is to the classification point, the more uniform the particle diameter of the fine powder obtained.
The term "classification point" herein used refers to the particle diameter at the intersection of the particle diameter distribution curves for collected fine and coarse powder particles.
In this type of conventional classifier, micron-order classification is already possible. But there is a demand to further reduce the powder classification point in fields where the product is fine powder. Also it is required to reduce the maximum particle size of fine powder.
It is known that the classification point of powder is affected by the whirling speed of fluid in the classifying chamber 32, and the classification point can be decreased by increasing the whirling speed of fluid.
The whirling speed of fluid in the classifying chamber 32 can be increased by increasing the supply pressure of the fluid mixture introduced into the classifying chamber 32 in a whirling flow. But the supply pressure cannot be increased without a limit.
Thus, in this type of classifier, the angles of the louvers 33 are adjusted with the supply pressure of fluid mixture kept constant to change the flow rate of secondary air introduced into the classifying chamber 32, thereby adjusting the whirling speed of fluid in the chamber.
Based on the knowledge that there is a correlation between the whirling speed of powder in the classifying chamber and the classification point, the inventors thought that the whirling speed may have some influence on the maximum particle diameter, and measured the tangential flow speed at various positions in the classifying chamber 32. The results are shown in FIG. 7. For the test, the classifier shown in FIG. 6 was used.
The dimensions of the classifier used for measurement are shown in Table 1.
TABLE 1 Symbol D.sub.3 D.sub.4 D.sub.5 d.sub.2 H.sub.2 .alpha., .beta. Size(mm) 280 195 270 60 20 24.degree.
In the table,
D3=inner diameter of the classifying chamber 32 PA1 D4=outer diameter of the classifying cover 30 PA1 D5=outer diameter of the classifying plate 31 PA1 d2=inner diameter of the fine powder discharge tube 36 PA1 H2=height of the louvers 33 PA1 .alpha.2, .beta.2=inclination angles of the conical bottom surface of the classifying cover 30 and the conical top surface of the classifying plate 31
Measurement was made with high-pressure (2 kg/cm.sup.2) air being blown into the powder supply tube 34 and a suction force of -0.3 kg/cm.sup.2 applied to the fine powder discharge tube 36.
The velocity curves (I) to (IV) in FIG. 7 represent whirling speeds when the gap between louvers 33 was set at 1 mm, 3 mm, 5 mm and 7 mm, respectively.
As will be apparent from FIG. 7, in this type of classifier, the fluid whirling speed is extremely high at a point slightly spaced from the center of the classifying chamber 32 (that is, a point slightly offset inwardly from the inner surface of the fine powder discharge tube 36), and decreases gradually toward the inner surface of the classifying chamber 32.
In the conventional classifier, because the powder supply port 35 formed along the outer edge of the classifying cover 30 is an area where the whirling speed is relatively low, the whirling speed of powder supplied into the classifying chamber 32 through the powder supply port 35 is low. It is thus impossible to impart a sufficient dispersing and whirling force to the powder.
Coarse particles are thus likely to mix into fine particles, increasing the maximum particle diameter of fine powders.
Also, in the conventional classifier, the classifying chamber 32 for classifying powder into fine and coarse particles by centrifugal force has a cylindrical inner surface 38 above the louvers 33. Therfore, so that powder whirling in the outer circumference of the classifying chamber 32 tends to dwell, adhere to the cylindrical inner surface 38, and build up without being sufficiently acted on by secondary air introduced into the classifying chamber 32 through the gaps between the louvers 33. This tendency is especially remarkable if the powder particle diameter is small because such small-diameter powder particles tend to be more strongly pushed against the cylindrical inner surface 38 by centrifugal force. This reduces the recovery rate of classified powder. Also, due to the adhesion of powder, the shape of the classifying chamber tends to change, making it impossible to operate the classifier stably with a constant classification point.
An object of the present invention is to reduce the classification point of powder in a classifier of the above-described type.
Another object of this invention is to achieve a stable operation with a constant classification point, and to increase the recovery rate of classified powder.
A further object of this invention is to reduce the maximum particle diameter of powder as a product.